That video is obviously not in a 16:9 aspect ratio. 16:9 video would only have black bars on the left and right side but not on the top and bottom. If you played the video from that photo on an iPhone 8 (Plus) it would have an even more pronounced letterbox effect.
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Yes, you are right, that one isn't 16:9 but I was talking about 16:9 video and every TV show nowadays (and videos on YouTube) is 16:9. The iPhone 8 / Plus screen is 16:9. TV panels are also 16:9. Coincidence? No, it's what offers the best viewing experience to date, that's why they happen to be in that aspect ratio.
Edit: the video in that example is not 16:9 but I don't like the fact that 1) the iPhone X cannot play 16:9 fullscreen without borders and 2) even when watching a movie, let's say in 1.85:1 format, it will be surrounded by big black borders which is not a good experience on an edge to edge display at all. Yes, you can zoom in but then there's that distracting notch covering content.
Edit #2: I'm not saying, watching video on an iPhone 8/Plus is perfect or a lot better, but I just cannot see the advantage of the iPhone X's edge to edge display in most scenarios. Ok, it has a big display but is more pocketable than an iPhone Plus and that's nice but the experience isn't just good enough for the very high price Apple likes us to pay for it. It's not that futuristic, edge to edge phone we were waiting for.
To me it's a peculiar and compromised phone with a weird, distracting notch that makes you zoom out videos and photos so that at the end of the day you'll have to ask yourself why you spent so much money for an iPhone with an edge to edge display which in reality cannot be experienced when watching videos on YouTube, TV shows on Netflix, looking at photos, playing games, surfing the net while in landscape mode. Except when you embrace the notch that hides (!) content and even the scrollbar on websites.